A so-called two component type heat-sensitive recording material using a color-forming reaction of a colorless or slightly colored electron-donating dye precursor with an electron-accepting compound is disclosed in Japanese Patent Publication Nos. 14039/70 and 4160/68. This type of two-component color-forming heat-sensitive recording material is prepared by dispersing a colorless or slightly colored electron-donating dye precursor and an electron-accepting compound into a fine particle state, mixing a binder therewith so that the electron-donating dye precursor and the electron-accepting compound are separated, and coating the resulting mixture on a support. Upon heating, these heat-sensitive color-forming compounds melt, contact each other, and thus participate in a color-forming reaction whereby recording takes place.
Such two-component type color-forming heat-sensitive recording materials are advantageous in that: (1) primary coloration takes place and, therefore, color development is unnecessary; (2) paper quality is similar to paper used for ccnventional types of recording; (3) handling is easy; (4) color density of the resulting images is high; and (5) upon color formation, various hues can be obtained. Accordingly, this type of recording material is very attractive from an economic standpoint. Therefore, this type of two-component color-forming heat-sensitive recording material was become widely used in practice recently, particularly in the fields of facsimile transmissions, recorders and printers. With such increasingly wide usage in the field of facsimile transmissions, the recording rate has also increased significantly in recent years. Thus, in view of this tendency to increase facsimile recording rates for practical and economical reasons, a strong demand has arisen for heat-sensitive recording materials which have a short pulse, that is, the ability to undergo color formation with a low energy input. In other words, improvement of heat reactivity of the recording materials has long been desired.
On the other hand, this type of heat-sensitive recording material has a defect in that due to primary coloration, a reaction between an electron-donating dye precursor and an electron-accepting compound takes place not only upon heating, but also upon contact with certain solvents and the like.
This is because all components of the heat-sensitive recording materials are organic substances and have high solubility in solvents such as ethanolamines, ethylene glycols, etc., whereby a reaction takes place in the solvent. Therefore, when such heat-sensitive recording materials are brought into contact with stationeries containing solvents such as water-soluble ink pens (felt pens), oil-soluble ink pens (felt pens) or fluorescent pens, diazo developing solutions, bonds, pastes, etc., white areas of the recording material become colored and printed areas disappear, thereby remarkably reducing the commercial value of the material.
It has hitherto been suggested that a solvent-resistant protective layer can be provide on a heat-sensitive color-forming layer to eliminate the above problems, as disclosed in Japanese Patent Publication No. 27880/69 and Japanese Patent Application (OPI) Nos. 30437/73, 31958/73 and 111729/84 (corresponding to U.S. Pat. No. 4,583,103) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Various binders used for such a protective layer are known. Heat-sensitive recording materials using alkali salts of a copolymer of styrene and maleic acid and alkali salts of a copolymer of styrene and maleic acid ester as binders for a protective layer tend to have reduced coloration on white (background) areas and a reduced occurrence of image disappearance on printed areas when they are brought into contact with a fluorescent pen containing solvents such as triethanolamine, triethylene glycol, etc. However, this type of heat-sensitive recording material is generally of poor water resistivity, and has certain undesirable defects, e.g., when a heat-sensitive recording paper comes into contact with water and is then placed in contact with the other recording paper, they may be adhered. Thus, when printing takes place using a thermal head, sticking occurs. Hence, still further improvements in protective layers and, in particular, binders therefor, for this type of heat-sensitive recording material are necessary to overcome these types of problems.